A User Equipment (UE), also known as a recipient, a mobile station, wireless terminal and/or mobile terminal is enabled to communicate wirelessly in a wireless communication system, sometimes also referred to as a cellular radio system or a wireless communication network. The communication may be made, e.g., between UEs, between a UE and a wire connected telephone and/or between a UE and a server via a Radio Access Network (RAN) and possibly one or more core networks. The wireless communication may comprise various communication services such as voice, messaging, packet data, video, broadcast, etc.
The UE/recipient may further be referred to as mobile telephone, cellular telephone, computer tablet or laptop with wireless capability, etc. The UE in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another UE or a server.
The wireless communication system covers a geographical area which is divided into cell areas, with each cell area being served by a radio network node, or base station, e.g., a Radio Base Station (RBS) or Base Transceiver Station (BTS), which in some networks may be referred to as “eNB”, “eNodeB”, “NodeB” or “B node”, depending on the technology and/or terminology used.
Sometimes, the expression “cell” may be used for denoting the radio network node itself. However, the cell may also in normal terminology be used for the geographical area where radio coverage is provided by the radio network node at a base station site. One radio network node, situated on the base station site, may serve one or several cells. The radio network nodes may communicate over the air interface operating on radio frequencies with any UE within range of the respective radio network node.
In some radio access networks, several radio network nodes may be connected, e.g., by landlines or microwave, to a Radio Network Controller (RNC), e.g., in Universal Mobile Telecommunications System (UMTS). The RNC, also sometimes termed Base Station Controller (BSC), e.g., in GSM, may supervise and coordinate various activities of the plural radio network nodes connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Spécial Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE)/LTE-Advanced, radio network nodes, which may be referred to as eNodeBs or eNBs, may be connected to a gateway, e.g., a radio access gateway, to one or more core networks.
In the present context, the expressions downlink (DL), downstream link or forward link may be used for the transmission path from the radio network node to the UE. The expression uplink (UL), upstream link or reverse link may be used for the transmission path in the opposite direction, i.e., from the UE to the radio network node.
Furthermore, in order to divide forward and reverse communication channels on the same physical communications medium, when communicating in the wireless communication system, a duplexing method may be applied such as, e.g., Frequency-Division Duplexing (FDD) and/or Time-Division Duplexing (TDD). The FDD approach is used over well separated frequency bands in order to avoid interference between uplink and downlink transmissions. In TDD, uplink and downlink traffic are transmitted in the same frequency band, but in different time intervals. The uplink and downlink traffic is thus transmitted separated from each other, in the time dimension in a TDD transmission, possibly with a Guard Period (GP) in between uplink and downlink transmissions. In order to avoid interference between uplink and downlink, for radio network nodes and/or UEs in the same area, uplink and downlink transmissions between radio network nodes and UEs in different cells may be aligned by means of synchronization to a common time reference and use of the same allocation of resources to uplink and downlink.
The prior art LTE-Advanced system supports carrier aggregation, where the communication between the radio network node (eNodeB) and the UE is facilitated by means of concurrent usage of multiple component carriers in the downlink and/or the uplink. Component carriers may be located contiguously or discontiguously in frequency within a frequency band or may even be located in different frequency bands. Hence, carrier aggregation improves the spectrum utilization for the network operator and allows higher data rates to be provided. Although carrier aggregation is defined both for FDD and TDD, UEs in the prior art system do not operate on FDD and TDD carriers simultaneously, hence there is no carrier aggregation utilizing carriers with different duplexing methods. Since network operators may be in possession of both FDD and TDD carriers, it is however desirable to extend the principle to carrier aggregation of FDD and TDD carriers.
Contemporary wireless systems, such as the 3GPP LTE, utilise packet based transmissions. Upon reception of a data packet, the UE transmits Hybrid Automatic Repeat Request (HARQ) messages to the radio network node. These messages may for example comprise an acknowledgement (ACK) or a negative ACK (NACK). New packet transmission or packet retransmissions may subsequently be initialized by the transmitting part once the HARQ feedback is obtained. HARQ feedback signalling will require uplink transmission resources and it is essential to minimize the amount of time-frequency resources to be allocated for HARQ feedback since unused uplink resources may be utilised e.g. for transmitting user data instead. A further problem is to assign a set of uplink resources assuring that there are no uplink resource conflicts, i.e., each recipient/UE must be assigned a set of unique uplink resources for HARQ.
HARQ feedback is sent in the UL in response to a Physical Downlink Shared Channel (PDSCH) scheduled by a Physical Downlink Control Channel (PDCCH)/Enhanced-PDCCH (EPDCCH), a semi-persistently scheduled (SPS) PDSCH or a PDCCH/EPDCCH indicating SPS release. Three feedback states may be used; ACK, NACK and Discontinuous Transmission (DTX). Sometimes NACK may be merged with DTX to a joint state NACK/DTX. In that case, the radio network node cannot discriminate between the NACK and DTX and would, if there was a scheduled PDSCH, need to perform a retransmission. This also precludes using incremental redundancy for the retransmission. DTX refers to discontinuous transmission, which happens if the UE did not receive any PDSCH, e.g., if it missed receiving a transmitted PDCCH/EPDCCH, or if there was no transmitted PDCCH/EPDCCH or PDSCH.
Thus, when applying FDD, the same numbers of uplink and downlink subframes are available during a radio frame, why HARQ feedback may be provided in an uplink subframe for each received downlink subframe and vice versa. In other words, every downlink subframe can be associated to a specific later uplink subframe for feedback generation in way that this association is one-to-one, i.e., to each uplink subframe is associated exactly one downlink subframe. However, in TDD the number of uplink and downlink subframes may be different in some configurations, for example comprising more downlinks subframes than uplink subframes, as illustrated in FIG. 1A.
Generally, one HARQ message is associated with each downlink subframe in TDD, since a data packet (e.g., transport block in LTE) is transmitted in one subframe. This implies that HARQ messages from multiple downlink subframes may need to be transmitted in a single uplink subframe, which requires allocation of multiple unique uplink resources for HARQ. In such scenario, comprising, e.g., four downlink subframes for each uplink subframe, the receiver has to provide HARQ feedback for all the four downlink subframes in one single uplink subframe, as illustrated in FIG. 1B. When doing so, the HARQ feedback may occupy a significant amount of the uplink communication resources. Hence, in particular for TDD, where an uplink subframe may comprise HARQ messages for many users and from multiple subframes, it is essential that the network nodes can make an efficient uplink resource assignment. This becomes particularly important when there are fewer uplink subframes than downlink subframes in a radio frame, since the amount of reserved uplink control channel resources impacts the available resources for data transmission.
In some access technologies such as, e.g., LTE-Advanced, carrier aggregation may be performed by receiving/transmitting on a set of serving cells, wherein a serving cell comprises at least a DL component carrier and possibly an UL component carrier. Here, the notion of cell may not refer to a geometrical area, rather it is should be regarded as logical concept. A UE is always configured with a primary serving cell (PCell) and additionally also with secondary serving cells (SCells). The Physical Uplink Control Channel (PUCCH) is always transmitted on the PCell.
Concerning carrier aggregation, one major issue concerns the uplink feedback. For downlink carrier aggregation, the UE will provide HARQ feedback in the PUCCH transmitted on the primary cell, including ACK and NACK messages corresponding to the received transport blocks in the downlink. For spatial multiplexing techniques, up to 2 transport blocks may be transmitted in a downlink subframe on a component carrier. For FDD, each downlink subframe can be associated with one unique uplink subframe, wherein the PUCCH is transmitted. For TDD, the number of downlink subframes may be larger than the number of uplink subframes, thus several downlink subframes may be associated with one unique uplink subframe. Hence, an uplink subframe may need to carry HARQ information corresponding to multiple downlink subframes in the PUCCH in TDD.
It is thus a problem to allocate uplink transmit resources for HARQ feedback in carrier aggregation of TDD and FDD, such that resources are unique for different subframes while minimizing the uplink resource overhead.
Several PUCCH signalling formats exist which may carry HARQ feedback in LTE-Advanced. One type of PUCCH format utilizes Quadrature Phase-Shift Keying (QPSK) or Binary Phase-Shift Keying (BPSK) modulated sequences such as i.e., format 1a/1b. When extended with selection from multiple (up to 4) sequences (i.e., format 1b with channel selection), 4 HARQ-ACK bits may be conveyed. These formats are used both with and without carrier aggregation and are able to provide HARQ feedback for up to 2 component carriers, which is the most practical case in reality considering the UE complexity. Another type of PUCCH format is DFT spread OFDM (i.e., format 3) which is capable of carrying more HARQ feedback (e.g., 20 HARQ-ACK bits). The UE is configured by the radio network node whether it may use PUCCH format 3 or the PUCCH format 1b based schemes. However, PUCCH format 3 may not be needed if only 2 component carriers are aggregated.
For TDD, the frame structure comprises, in addition to normal subframes, special subframes which contain a first part for downlink transmissions; Downlink Pilot Time Slot (DwPTS), a second part for Guard Period (GP) and last part for uplink transmissions; Uplink Pilot Time Slot (UpPTS), see FIG. 1C. The duration of the different parts may vary and may be configurable by the system.
A downlink subframe is illustrated in FIG. 1D and an uplink subframe is illustrated FIG. 1E.
Thus, for TDD, M=1, 2, 3 or 4 downlink subframes may be associated with an uplink subframe. For aggregating 2 component carriers with spatial multiplexing on each carrier, there may thus be up to 4*2*2=16 HARQ-ACK bits in one subframe, which cannot be accommodated using PUCCH format 1b with channel selection. Therefore, various forms of HARQ information compression techniques are utilised to reduce the number of HARQ-ACK bits. For example, a logical AND operation among HARQ-ACK bits may be performed either among transport blocks (spatial bundling) in a subframe, across subframes (time-domain bundling) or across component carriers. A drawback is that a bundled NACK implies that a retransmission has to be performed for all transport blocks in the bundle. The consequence would therefore be lower throughput and less spectral efficiency. Bundling is predominately a problem for TDD, since for FDD, at most 4 HARQ-ACK bits need to be accommodated (assuming 2 component carriers with spatial multiplexing), which can be handled with format 1b with channel selection without bundling.
For TDD, a component carrier is configured with 1 out of 7 UL-DL configurations, defining the transmission direction of the subframes in the radio frame. A radio frame comprises downlink subframes, uplink subframes and special subframes. The special subframes contain one part for downlink transmission, a guard period and one part for uplink transmission. The number of downlink subframes, M, for which an uplink subframe may transmit HARQ feedback is dependent on the TDD UL-DL configuration as well as the index of the specific uplink subframe. In practice, the same UL-DL configuration has to be used in neighboring cells in order to avoid UE-to-UE and eNodeB-to-eNodeB interference. Thus it is not straightforward to reconfigure the UL-DL configuration, e.g., in order to adapt to the traffic load. However, LTE-Advanced also allows the possibility to dynamically change the direction of a subframe. This may be denoted as a flexible subframe. For example, an indication may be given to UEs that are capable of such dynamic subframe direction change, to utilise a subframe for downlink transmission even though it is an uplink subframe according to the cell-specific UL-DL configuration. If an uplink subframe has been used as a flexible subframe for downlink transmission, there is no associated uplink subframe for the corresponding HARQ information according to the cell-specific UL-DL configuration and such UEs may follow a different HARQ timing (e.g., that of another reference TDD UL-DL configuration) than that of the given UL-DL configuration.
The PDCCH/EPDCCH comprise the Downlink Control Information (DCI) related to the PDSCH transmission. This comprises, e.g., HARQ process number (3 bits for FDD and 4 bits for TDD). For TDD there is also a Downlink Assignment Index (DAI) of 2 bits. For DCI containing downlink assignments, the DAI works as an incremental counter denoting the accumulative number of PDCCHs/EPDCCHs with assigned PDSCH transmission(s) and PDCCH/EPDCCH indicating SPS release, up to the present subframe of the bundling window. For DCI containing uplink grants, the DAI indicates the total number of subframes with PDSCH(s) and PDCCHs/EPDCCHs indicating SPS release that were transmitted during the bundling window of M downlink subframes. With the DAI information, the UE may be able to detect whether it has missed receiving any PDSCH or PDCCH/EPDCCH (except the last one) and if it may correspondingly transmit a bundled ACK or NACK.
PUCCH format 1b with channel selection assumes that a set of channels (i.e., sequences, or PUCCH resources) are reserved for the UE and as a way of encoding the HARQ message, it selects one of the channels, which is then modulated with a QPSK symbol. With up to 4 channels reserved, at most 4 HARQ-ACK bits (i.e., 16 unique states of HARQ information) can be provided. The PUCCH resource reservation can be performed implicitly by a mapping from the time-frequency resources occupied by the PDCCH/EPDCCH to the PUCCH resources. Implicit resource reservation is used when the PDCCH/EPDCCH is located on the PCell, either scheduling the PDSCH on the PCell or on the SCell by so called cross-carrier scheduling. Explicit resource reservation is utilised if the PDCCH/EPDCCH is located on the SCell or for SPS transmission of PDSCH on the PCell, for which there is no PDCCH/EPDCCH. For explicit resource reservation, 2 bits in the PDCCH/EPDCCH indicate 1 or 2 higher-layer configured resources which may be reserved. These 2 bits are obtained by reusing the 2 bits of the Transmit Power Control (TPC) field related to the PUCCH. Consequently, TPC commands cannot be signaled in the DCI when the PDCCH/EPDCCH is transmitted on the SCell.
For TDD, with a capability of transmitting only 4 HARQ-ACK bits (i.e., 16 HARQ states), it is not possible to represent all combinations of ACK, NACK and DTX states for 2 component carriers when M>1. Therefore, spatial bundling is used when M>1. However, when M>2, spatial bundling is not sufficient and a form of time-domain bundling is also performed and separate tables are given for M=3 and M=4. The time-domain bundling in this case corresponds to prioritizing HARQ states representing subframes having consecutive ACKs and associating such states with unique channel and modulation combinations.
In the uplink, the UE is also able to send a Scheduling Request (SR) when it has uplink data to transmit. The SR may be provided on a higher-layer configured channel (i.e., sequence or PUCCH resource). At most two bits may be conveyed on the SR resource, assuming QPSK modulation. If the UE is supposed to transmit HARQ information together with the SR, channel selection cannot be performed and the HARQ-ACK bits are bundled such that at most 2 bundled bits remain. This amounts to selecting only a modulation symbol (i.e., QPSK symbol representing the 2 bits) and transmitting it on the allocated SR resource. For FDD, this is facilitated by spatial bundling. Moreover, the spatial bundling is always performed such that only 1 HARQ-ACK bit is transmitted per serving cell, even though 2 non-bundled HARQ-ACK bits could be transmitted. That is, even if there is no transmission on the SCell (PCell), spatial bundling is performed on the HARQ-ACK bits on the PCell (SCell). This is to avoid the case where the radio network node has performed a transmission (and is thus expecting bundled HARQ information) while the UE missed the transmission. For TDD, the bundling comprises feeding back the number of ACKs among all the transport blocks, subframes and component carriers. However, this bundling mapping is not unique since 10 such states are associated with only 2 bundled HARQ-ACK bits. Therefore, the radio network node may not easily be able to determine which transmissions that were received correctly and the probability for retransmission of all transport blocks is non-negligible.
In order to minimize the complexity in the UE, it would be beneficial to support downlink carrier aggregation of 1 FDD carrier and 1 TDD carrier utilizing HARQ feedback by format 1b with channel selection. Current HARQ feedback with PUCCH format 1b with channel selection for TDD involves significant HARQ bundling which should be avoided and especially to avoid introducing bundling for the FDD carrier in a joint feedback method.
It is a problem to define a method for simultaneous joint HARQ feedback for an FDD carrier and a TDD carrier.
It is a further problem to reduce the amount of bundling when a Scheduling Request (SR) is transmitted with HARQ information. Hence, it is a general problem to assure that there is a reasonable trade-off between control channel overhead and performance.